Method and system for hybrid integration of a tunable laser and a phase modulator

ABSTRACT

A tunable laser includes a substrate comprising a silicon material and a gain medium coupled to the substrate. The gain medium includes a compound semiconductor material. The tunable laser also includes an optical modulator optically coupled to the gain medium, a phase modulator optically coupled to the optical modulator, and a waveguide disposed in the substrate and optically coupled to the gain medium. The tunable laser further includes a first wavelength selective element characterized by a first reflectance spectrum and disposed in the substrate, a second wavelength selective element characterized by a second reflectance spectrum and disposed in the substrate, an optical coupler disposed in the substrate and joining the first wavelength selective element, the second wavelength selective element, and the waveguide, and an output mirror.

CROSS-REFERENCES TO RELATED APPLICATIONS

This present application is a continuation-in-part of U.S. patentapplication Ser. No. 12/903,025, filed on Oct. 12, 2010, which claimspriority to U.S. Provisional Patent Application No. 61/251,143, filed onOct. 13, 2009, the disclosures of which are hereby incorporated byreference in their entirety for all purposes. U.S. patent applicationSer. No. 12/903,025 was filed concurrently with 12/902,621, thedisclosure of which is hereby incorporated by reference in its entiretyfor all purposes.

BACKGROUND OF THE INVENTION

Advanced electronic functions such as photonic device bias control,modulation, amplification, data serialization and de-serialization,framing, routing, and other functions are typically deployed on siliconintegrated circuits. A key reason for this is the presence of a globalinfrastructure for the design and fabrication of silicon integratecircuits that enables the production of devices having very advancedfunctions and performance at market-enabling costs. Silicon has not beenuseful for light emission or optical amplification due to its indirectenergy bandgap. This deficiency has prevented the fabrication ofmonolithically integrated opto-electronic integrated circuits onsilicon.

Compound semiconductors such as indium phosphide, gallium arsenide, andrelated ternary and quaternary materials have been extremely importantfor optical communications, and in particular light emitting devices andphotodiodes, because of their direct energy bandgap. At the same time,integration of advanced electrical functions on these materials has beenlimited to niche, high-performance applications due to the much highercost of fabricating devices and circuits in these materials.

Thus, there is a need in the art for improved methods and systemsrelated to hybrid integration of silicon and compound semiconductordevices.

SUMMARY OF THE INVENTION

Embodiments of the present invention relate to hybrid-integrated siliconphotonics. More particularly, embodiments of the present inventionrelate to an apparatus and method of hybrid integration of compoundsemiconductor chips with tuning elements monolithically integrated ontoa silicon base and the like.

According to an embodiment of the present invention, techniques relatedto photonic integration are provided. Merely by way of example,embodiments of the present invention have been applied to methods andsystems for fabricating and operating a tunable laser utilizing a hybriddesign. More particularly, an embodiment of the present inventionincludes a hybrid system including a semiconductor laser devicefabricated in a first material system and a wavelength tuning devicefabricated in a second material system. In some embodiments, the tunablelaser is fabricated using bonding methodology described in related U.S.patent application Ser. No. 12/902,621. However, the scope of thepresent invention is broader than this application and includes otherphotonic systems.

According to an embodiment of the present invention, a tunable pulsedlaser is provided. The tunable pulsed laser includes a substratecomprising a silicon material and a gain medium coupled to thesubstrate. The gain medium includes a compound semiconductor material.The tunable pulsed laser also includes an optical modulator opticallycoupled to the gain medium, a phase modulator optically coupled to theoptical modulator, and a waveguide disposed in the substrate andoptically coupled to the gain medium. The tunable pulsed laser furtherincludes a first wavelength selective element characterized by a firstreflectance spectrum and disposed in the substrate, a second wavelengthselective element characterized by a second reflectance spectrum anddisposed in the substrate, an optical coupler disposed in the substrateand joining the first wavelength selective element, the secondwavelength selective element, and the waveguide, and an output mirror.

According to another embodiment of the present invention, a method ofoperating a tunable pulsed laser is provided. The method includes tuninga first modulated grating reflector and tuning a second modulatedgrating reflector. The first modulated grating reflector ischaracterized by a first reflectance spectra including a first pluralityof reflectance peaks and the second modulated grating reflector ischaracterized by a second reflectance spectra including a secondplurality of reflectance peaks. The method also includes generatingoptical emission from a gain medium comprising a compound semiconductormaterial, waveguiding the optical emission to pass through an opticalcoupler, reflecting a portion of the optical emission having a spectralbandwidth defined by an overlap of one of the first plurality ofreflectance peaks and one of the second plurality of reflectance peaks,and amplifying the portion of the optical emission in the gain medium.The method further includes transmitting a portion of the amplifiedoptical emission through an output mirror, optically modulating thetransmitted optical emission to form a pulsed optical output, and phasemodulating the pulsed optical output.

According to yet another embodiment of the present invention, bothamplitude and phase of the optical emission are modulated, withinformation contained on both the real (amplitude) and imaginary (phase)portions of the optical signal. The method additionally may includetransmitting the light through an optical device for SBS suppression.The method further includes the driving and monitoring of all opticaldevices on the chip with circuit elements on the chip.

Numerous benefits are achieved by way of the present invention overconventional techniques. For example, embodiments of the presentinvention provide methods and systems suitable for reducing the size andpower consumption of optical communications systems, relaxing therequirements for stringent temperature control of the devices, andimproving the laser linewidth through minimizing refractive indexfluctuations in the device. These and other embodiments of the inventionalong with many of its advantages and features are described in moredetail in conjunction with the text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a simplified plan view illustrating a hybrid integratedtunable laser according to an embodiment of the present invention;

FIG. 1B is a simplified cross-sectional view illustrating a hybridintegrated tunable laser according to a particular embodiment of thepresent invention;

FIG. 1C is a simplified cross-sectional view illustrating a hybridintegrated tunable laser according to a specific embodiment of thepresent invention;

FIG. 2A is a cross-sectional view at cross section A-A′ as illustratedin FIG. 1A;

FIG. 2B is a cross-sectional view at cross section B-B′ as illustratedin FIG. 1A;

FIG. 3A is a simplified perspective view of a waveguide includinggrating elements according to an embodiment of the present invention;

FIG. 3B is a simplified cross-sectional view at a high index portion ofthe waveguide illustrated in FIG. 3A according to an embodiment of thepresent invention;

FIG. 3C is a simplified cross-sectional view at a low index portion ofthe waveguide illustrated in FIG. 3A according to an embodiment of thepresent invention;

FIG. 3D is a contour plot illustrating a TE mode for the high indexportion of the waveguide illustrated in FIG. 3B;

FIG. 3E is a contour plot illustrating a TM mode for the high indexportion of the waveguide illustrated in FIG. 3B;

FIG. 3F is a contour plot illustrating a TE mode for the low indexportion of the waveguide illustrated in FIG. 3C;

FIG. 3G is a contour plot illustrating a TM mode for the low indexportion of the waveguide illustrated in FIG. 3C;

FIG. 4A illustrates a reflectance spectrum for a first modulated gratingreflector according to an embodiment of the present invention;

FIG. 4B illustrates a reflectance spectrum for a second modulatedgrating reflector according to an embodiment of the present invention;

FIG. 4C illustrates an overlay of the reflectance spectra shown in FIG.4A and FIG. 4B;

FIG. 4D illustrates constructive interference between the reflectancespectra shown in FIG. 4A and FIG. 4B;

FIG. 5A is a plot illustrating operating wavelength as a function oftemperature change according to an embodiment of the present invention;

FIG. 5B illustrates wavelength shifting of a reflectance spectrum as afunction of index of refraction according to an embodiment of thepresent invention;

FIG. 6 is a simplified flowchart illustrating a method of operating ahybrid integrated laser according to an embodiment of the presentinvention;

FIG. 7 is a simplified plan view illustrating a hybrid integratedtunable pulsed laser and a phase modulator according to an embodiment ofthe present invention; and

FIG. 8 is a simplified flowchart illustrating a method of operating atunable pulsed laser according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Hybrid integration on silicon is preferable for the commercialdeployment of optoelectronic integrated circuits. Silicon is apreferable material for electronic integration. Silicon technology hasadvanced such that extremely complex electronic functions can berealized very inexpensively. Silicon is also a good material forconstructing low loss optical waveguides. However, monolithicintegration of light generating or detecting functions has beenprevented in silicon because it is an indirect bandgap material.Conversely, compound semiconductor materials, including III-V materialssuch as indium phosphide are well suited for light generation anddetection because of their physical properties such as being directbandgap materials. These materials are complex material systems withsmall substrates and relatively (compared to silicon) low yields. Assuch, constructing devices with a high level of functionality iscurrently cost prohibitive.

Embodiments of the present invention relate to an apparatus and methodfor hybrid integration of compound semiconductor devices with tuningelements monolithically integrated onto a silicon base or similarmaterial. Throughout this specification, the term “compositeintegration” can be used interchangeably with the term “hybridintegration.” Preferably, hybrid or composite integration is the methodto overcome the specific deficiencies of silicon and compoundsemiconductors while capitalizing on their respective strengths.Embodiments of the present invention preferably utilize the complexelectronic functionality in available using silicon devices to minimizecost, and the optical functions (e.g., light generation and detection)available using III-V materials to form hybrid integrated systems. Someembodiments of the present invention remove functionality from the III-Vmaterial system and transfer such functionality to the silicon system toimprove system performance.

Embodiments of the present invention utilize photonic apparatusfabricated using compound semiconductor material systems that aremounted onto silicon integrated circuit platforms and the like.Embodiments of the present invention achieve photonic integration byutilizing a plurality of techniques and apparatus that do nothistorically rely on a direct energy bandgap, including, but not limitedto, waveguides, optical multiplexers, optical demultiplexers, opticalmodulators, and the like, that can be fabricated using silicon andsimilar materials. Embodiments of the present invention optionallyinclude, but are not limited to, methods of modifying the refractiveindex of silicon via current injection or local heating.

Embodiments of the present invention include, but are not limited to,optionally utilizing the laser devices that serve as the initial sourceof optical energy. In today's dense wavelength division multiplexing(“DWDM”) systems, the laser sources are typically fixed-wavelengthdistributed feedback lasers or tunable lasers. Tunable lasers preferablyprovide additional flexibility to the optical communications networkoperators. Some DWDM systems can use lasers with up to 80 differentwavelengths. A single tunable laser is capable of tuning to any of thosewavelengths. One tunable laser can be inventoried and used to replaceany of 80 fixed wavelength lasers, thereby reducing the requiredinventory levels and the associated costs.

The term “silicon” as used throughout this application includes but isnot limited to tetravalent nonmetallic elements and the like. The term“laser” as used throughout the specification includes but is not limitedto an acronym for light amplification by stimulated emission ofradiation; and/or an optical device that produces an intensemonochromatic beam of coherent light. The term “SOI” and/or “Silicon onInsulator” stands for, a type of substrate material as used throughoutthis specification includes but is not limited to grating and tuningtesting. The term “DWDM” and/or “Dense Wavelength Division Multiplexing”as used throughout this application includes but is not limited to atechnique utilized by the optical communications industry to maximizesystem bandwidth while minimizing capital expenditures and operationalexpenditures. These costs are minimized through the use of DWDMtechniques because the system operators can increase their systembandwidth simply by adding another optical wavelength as opposed toneeding to deploy additional optical fibers which usually requiressignificant expense. The term “bandgap” as used throughout thisapplication includes but is not limited to an energy range in a solidwhere no electron states exist; and/or the energy difference between thetop of the valence band and the bottom of the conduction band; and/orthe amount of energy required to free an outer shell electron from itsorbit about the nucleus to a free state; and/or any combination thereof.The term “photonic integration” as used throughout this applicationincludes but is not limited to the meaning to make into a whole or makepart of a whole multiple functions and reduce packaging size by an orderof magnitude, for example, while matching the performance of a subsystembuilt with discrete components. The term “gain media” andinterchangeably “gain chip” as used throughout this application includesbut is not limited to the source of optical gain within a laser. Thegain generally results from the stimulated emission of electronic ormolecular transitions to a lower energy state from a higher energystate. The term “InP” or “Indium Phosphide”, as used throughout thisapplication is used interchangeably with the phrase “III-V compoundsemiconductor”.

FIG. 1A is a simplified plan view illustrating a hybrid integratedtunable laser according to an embodiment of the present invention. Asillustrated in FIG. 1A, laser 10 is a hybrid integrated structureincluding both active and passive elements disposed on or fabricated ina silicon substrate 22. Although a silicon substrate 22 is illustrated,this is intended to include a variety of semiconductor devicesfabricated using the silicon material system. Such devices include CMOScircuitry, current sources, laser drivers, thermal system controllers,passive optical elements, active optical elements, and the like.

Referring to FIG. 1A, a first modulated grating reflector 12 and asecond modulated grating reflector 14 are fabricated on the siliconsubstrate 22. Modulated grating reflectors 12 and 14 are preferablymodifiable to adjust the refractive index. The first modulated gratingreflector 12 and the second modulated grating reflector 14 are examplesof wavelength selective elements that are utilized according toembodiments of the present invention. The illustration of the use ofmodulated grating reflectors in FIG. 1A is not intended to limit thescope of the present invention but merely to provide examples ofwavelength selective elements. Other wavelength selective elements canbe utilized in embodiments of the present invention. As described morefully below, the wavelength selective elements can be sampled Bragggratings or sampled distributed feedback reflectors that provide a combof reflectance peaks having a variable comb spacing over a tunablewavelength range. Embodiments of the present invention are not limitedto these implementations and photonic crystals, etalon structures, MEMSdevices, ring resonators, arrayed-waveguide grating devices,Mach-Zehnder lattice filters, and the like can be employed as wavelengthselective elements. A benefit provided by the wavelength selectiveelements discussed herein is a reflection spectra including a single ormultiple peaks that can be shifted through the use of a controllableparameter such as current, voltage, temperature, mechanical force, orthe like.

As an example, heaters integrated into the silicon substrate can beutilized to locally change the temperature of the region surrounding themodulated grating reflectors and thereby, the index of refraction. Asdescribed more fully below, the ability to control the local index ofrefraction provides the functionality of varying the reflectivity of themodulated grating reflectors and the output wavelength of the hybridintegrated device.

Laser 10 further includes, but is not limited to, multimode interferencecoupler 16 and one or multiple phase adjustment sections 18. The phaseadjustment section 18 can also be referred to as a phase control regionthat provides for correction of phase misalignment between the output ofthe coupler section, which may be implemented through wavelengthselective devices (e.g., the grating sections) and the gain media 20. Inthe illustrated embodiment, the phase adjustment section 18 ispositioned between the multimode interference coupler 16 and the gainmedia 20, however, other embodiments locate this element in differentlocations providing the same or similar performance characteristics.

The coupler section, which may be implemented through the use of amultimode interference coupler, y-branch, or other method, splits andrecombines light from two or more tuning sections. The multimodeinterference coupler, which is based upon the principle that coherentlight launched from a waveguide (input waveguide) into a propagationsection will self image at periodic intervals, can be used toefficiently achieve n×m splitting ratios. In this instance, the designis optimized for a 1×2 split but other splitting ratios may be employedin the case where there are either multiple gain chips or more than 2tuning arms. An advantage provided by the illustrated device is thatcoherent light returning from the tuning arms, where the phaserelationship of the light is fixed, can be coupled back into the launchwaveguide with minimal excess loss. In order to ensure that theinterference pattern of the returning light has maximum overlap with theinput waveguide, a phase adjustment section may be implemented in one ormore of the branch arms. In addition to phase adjustment in the brancharms, a phase adjustment section 18 is utilized in the waveguide sectionleading from the coupler 16 to the gain chip 20. This phase adjustmentsection, which can be implemented though a device such as a heater orcurrent injection electrode, which changes the refractive index in thewaveguide layer under the device, serves to provide an overlap betweenthe cavity modes of the device and the grating mode selected by tuningsection.

As illustrated in FIG. 1A, gain media 20 (also referred to as a gainchip) fabricated using a compound semiconductor material system isintegrated with the silicon substrate 22 in a hybrid configuration. Thecompound semiconductor material, which is direct bandgap, providesoptical gain for the laser device. The hybrid integration or attachmentof the gain media (and/or other compound semiconductor elements) to thesilicon substrate can be provided in one or several manners. In aparticular embodiment, the hybrid integration is performed using themethods and systems described in the related application reference inparagraph [0002]. In addition to gain media, absorptive media fabricatedusing compound semiconductor materials can be integrated with thesilicon substrate. Embodiments of the present invention integrate III-Vdevices and structures acting as gain and/or absorption regions withsilicon photonics elements in which optical and/or electricalfunctionality is provided. The silicon photonic elements may includeCMOS circuitry and the like. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives.

As discussed in more detail in relation to FIGS. 4-4D, modulated gratingreflectors 12 and 14 provide feedback at one end of the laser 10.Feedback in the form of a front facet reflector is provided by a lowreflectance coating (e.g., a dielectric coating with a reflectance of afew percent, for example, ˜1-10%) applied to the gain media on surface21. Alternatively, a distributed feedback (e.g., a grating) structurecould be integrated into the silicon substrate to provide feedback forthe laser cavity. In another embodiment, a low reflectance coating isapplied to a surface of the silicon substrate. One of ordinary skill inthe art would recognize many variations, modifications, andalternatives. As illustrated in FIG. 1A, optical functionality otherthan optical gain has been transferred from the III-V materials in whichit is typically located and integrated into the silicon materials,thereby increasing device yield in comparison with designs that arefully integrated in III-V materials. In the illustrated embodiment, thetunable reflective sections (also referred to as wavelength selectivedevices) and other optical functions are performed in the siliconmaterial.

FIG. 1A also illustrates heater element 26 and temperature sensor 28associated with first modulated grating reflector 12 and heater element27 and temperature sensor 29 associated with the second modulatedgrating reflector. In an embodiment, the heater element can be a thinfilm resistor formed through the vacuum deposition of a material such asW, NiCr, TaN, WSi, RuO₂, PbO, Bi₂Ru₂O₇, Bi₂Ir₂O₇, cobalt salicide, orthe like.

In an embodiment, the temperature sensor can be a resistive thermaldevice (RTD), a thermocouple, a p-n junction, or the like. By flowing acurrent through the heaters, the temperature of the region surroundingthe modulated grating reflectors can be modified in order to modify theindex of refraction and the reflectance profile as a result. Phaseadjustment section 18, which also may use the temperature dependence ofthe refractive index to control the effective optical length and therebythe phase of light, is also provided with a heater and a temperaturesensor to provide similar functionality and wavelength tunability.

Some embodiments of the present invention utilize thermal tuning toachieve index of refraction changes in the silicon-based modulatedgrating reflectors. One of the benefits available using thermal tuningis a significant reduction in the short time scale variations in indexof refraction that are produced using thermal tuning in comparison tothese variations achieved using current tuning in the InP or GaAsmaterial system. Such improvement in refractive index stability willresult in a laser linewidth significantly narrower than can be achievedusing other approaches. As will be evident to one of skill in the art,the stable tuning provided by embodiments of the present inventionenables use of the lasers described herein in DWDM applications andother applications utilizing precisely tuned lasers. As an example,advanced modulation techniques such as DQPSK can benefit from use of thelasers described herein.

The phase adjustment section operates through the modification of therefractive index of the waveguide section contained therein. Throughmodification of the refractive index, the phase angle of the lightexiting the phase adjustment device relative the input phase angle canbe precisely controlled. This allows the alignment of laser cavity modeswith grating modes. In the illustrated embodiment, the phase adjustmentdevice 18 includes a heater 19 and a temperature sensor (e.g., an RTD)17.

FIG. 1B is a simplified cross-sectional view illustrating a hybridintegrated tunable laser according to a particular embodiment of thepresent invention. As illustrated in FIG. 1B, direct coupling betweenthe waveguide in the gain media and the waveguide in the silicon layeris utilized. The heater element and the temperature sensor (e.g., anRTD) are illustrated for the phase adjustment section as well as themodulated grating reflector sections. An encapsulant is illustrated overthe modulated grating reflector sections. The encapsulant provides forelectrical isolation among other features.

FIG. 1C is a simplified cross-sectional view illustrating a hybridintegrated tunable laser according to a specific embodiment of thepresent invention. The structure illustrated in FIG. 1C is similar tothat illustrated in FIG. 1B except that evanescent coupling between thewaveguide in the gain media and the waveguide in the silicon layer isutilized.

Referring to FIG. 1B, a Controlled Index Layer is illustrated that isnot necessarily the same as the index matching layer illustrated in FIG.2B. The controlled index layer can be used for mode shaping in thesilicon waveguide, for example, by using air, SiO₂ or the like.According to some embodiments of the present invention, a higher indexmaterial is utilized to broaden the mode in the silicon waveguide suchthat optical coupling to the gain media is improved. If the controlledindex layer is not an insulator, an encapsulant layer may also be usedbetween the heater metal and controlled index layer. As illustrated inFIGS. 1B and 1C, either direct coupling (also known as butt coupling) orevanescent coupling of the gain media to the silicon waveguide may beused.

Referring to FIG. 1C, the optical coupler, which may be a device such asa MMI (multimode interference coupler) is illustrated. In someembodiments, an MMI can be formed using an unguided propagation region.Additionally, although not illustrated in FIGS. 1A-1C, a second phaseadjust region may be provided in one of the legs of the Y-branchedstructure in addition to the phase adjustment section illustrated at theoutput of the tuning section.

FIG. 2A is a cross-sectional view at cross section A-A′ as illustratedin FIG. 1A. The silicon substrate 22 is illustrated as well as asilicon-on-insulator (SOI) oxide layer 23 and an SOI silicon layer 24.In the embodiment shown, a portion of the SOI silicon layer has beenremoved using an etching or other process to provide a recessed regioninto which the gain chip has been inserted. Such etching may not beperformed in the case where evanescent coupling of the light from thegain chip into the silicon waveguide is used. The gain chip is bonded tothe silicon substrate in the embodiment illustrated in FIG. 2A using ametal/metal structural bond at locations 25 that provide an electricalbond between the hybrid elements. Additionally, a metal/semiconductor ora semiconductor/semiconductor bond is illustrated. Combinations of thesebonding techniques can be implemented as well. One of ordinary skill inthe art would recognize many variations, modifications, andalternatives.

FIG. 2B is a cross-sectional view at cross section B-B′ as illustratedin FIG. 1A. As will be evident to one of skill in the art, the opticalwaveguide in the gain chip will be coupled to an optical waveguide inthe SOI silicon layer. An index matching region is provided at theinterface between the gain chip and the SOI silicon layer to facilitatea high degree of optical coupling between the hybrid devices and toreduce or minimize parasitic reflections. The index matching region canbe filled with an appropriate index matching material, remain empty,have optical coatings applied to the surfaces of the hybrid devices asillustrated at facets 26 and/or 27, combinations thereof, or the like.

Referring once again to FIG. 1A first modulated grating reflector 12provides optical feedback creating a comb of reflected opticalwavelengths. Second modulated grating reflector 14 provides opticalfeedback characterized by a different optical period, thereby resultingin a variable set of reflected wavelengths. The two combs of wavelengthsare combined in optical coupler 16. The combs overlap and lasingpreferably occurs due to constructive interference. Optionally, wherethe combs do not overlap, lasing is preferably prevented due todestructive interference. Specific optical spectra of first modulatedgrating reflector 12 and/or second modulated grating reflector 14 can bemodified by varying the refractive index. The refractive index ispreferably modified by varying the temperature of the modulated gratingreflectors 12, 14 using a heating element. The amount of heating isoptionally monitored through use of an RTD element.

Phase adjustment is provided using phase adjustment region 18 tocompensate for small phase offsets between the reflection spectra fromfirst modulated grating reflector 12 and the second modulated gratingreflector 14. Embodiments of the present invention comprise functionalblocks that can be realized in a compound semiconductor such as indiumphosphide, and/or silicon and/or similar material. Embodiments of thepresent invention comprise tuning by modifying the refractive index ofthe silicon and the like, preferably using a thermal technique.

In embodiments of the present invention, the gain media, whichpreferably uses a direct-bandgap material, can be realized in a compoundsemiconductor material. Other embodiments of the present inventioninclude functional blocks that can be realized in silicon materialsystems. Embodiments of the present invention utilize a hybrid-approachthat is preferable for a variety of reasons that include, but are notlimited to: manufacturing components using methods that can result inhigh-yields at low cost; virtually unlimited levels of additionalintegration can be achieved because of the complexity of the III-Vmaterial system as compared to the Si material system, and the like.Therefore, embodiments of the present invention encompass substantiallyall necessary circuits to control the operation of the tunable laser andcan also be monolithically integrated with silicon-based devices.

It should be noted that while embodiments of the present invention havebeen implemented in relation to products produced by the semiconductorindustry, embodiments of the present invention are also useful inoptical communications networks for the telecommunications industry, theenterprise communications industry, high-performance computinginterconnects, back-plane optical interconnects, chip-to-chip opticalinterconnects, intra-chip optical interconnects, and the like. Inaddition to these communication applications, embodiments of the presentinvention also have applications in the medical device industry.

The following figures illustrate an analysis and applications ofwaveguides created in silicon using an SOI substrate with a silicondioxide cap layer. This material system is merely described by way ofexample and embodiments of the present invention can be implemented inother material systems.

FIG. 3A is a simplified perspective view of a waveguide according to anembodiment of the present invention. As illustrated in FIG. 3A, awaveguide structure is formed with a periodic variation in thickness ofone or more layers making up the waveguide. In the illustratedembodiment, the SOI silicon layer varies in thickness with a highportion having thickness H and a low portion having thickness H-h. Thewidth of the waveguide is W. For purposes of clarity, only the top twoSOI layers (i.e., the SOI oxide layer and the SOI silicon layer) areillustrated in FIGS. 3A-3C. FIG. 3B is a simplified cross-sectional viewat a high index portion of the waveguide illustrated in FIG. 3Aaccording to an embodiment of the present invention. FIG. 3C is asimplified cross-sectional view at a low index portion of the waveguideillustrated in FIG. 3A according to an embodiment of the presentinvention. It should be noted that the top SiO₂ layer shown in thesefigures may be replaced by another index-controlled layer such as air,TiO₂, SiC, ZnS, Nb₂O₅, HfO₂, ZrO₂. As will be evident to one of skill inthe art, the indexes of the various materials will impact the shape ofthe optical modes.

The waveguide structure was analyzed to determine an effective index forthe various sections of the waveguide. A vector EM mode solver was usedand applied to two different single mode ridge waveguides with twodifferent ridge heights. The effective indices n_(H) and n_(L) and modeprofiles could be extracted, then the full three-dimensional problem wasa one-dimensional problem, with the one-dimensional transfer matrixmethod efficiently simulating the multi-layer structures. The indexdifference created reflections that accumulated coherently over thelength result in differing reflectances versus wavelength.

FIG. 3D is a contour plot illustrating a TE mode for the high indexportion of the waveguide illustrated in FIG. 3B. FIG. 3E is a contourplot illustrating a TM mode for the high index portion of the waveguideillustrated in FIG. 3B. FIG. 3F is a contour plot illustrating a TE modefor the low index portion of the waveguide illustrated in FIG. 3C. FIG.3G is a contour plot illustrating a TM mode for the low index portion ofthe waveguide illustrated in FIG. 3C.

FIG. 4A illustrates a reflectance spectrum for a first modulated gratingreflector according to an embodiment of the present invention and FIG.4B illustrates a reflectance spectrum for a second modulated gratingreflector according to an embodiment of the present invention. Asillustrated in FIG. 4A, the grating structure includes a superstructuregrating (SSG) in which periodically modulated gratings provide acomb-like reflection spectrum. In these gratings, multiple elements ofperiodicity are provided such that the mode spacing associated with thegrating is overlaid with an envelope. The spacing between the modes ofthe comb will be a function of the height and other features of thegrating features formed in the waveguide.

As an example of an SSG, the reflectance spectrum illustrated in FIG. 4Awas obtained using the following 3-step modulated superstructure gratingparameters:

Duty cycles=[0.5 0.5 0.5]

Periods=[227.7 230 232.3] nm N_(sub)=[110 109 108]Λ_(S)=(25.047+25.07+25.088)=75.205 μm

n_(H)=3.3757; n_(L)=3.3709;Δn=n_(H)−n_(L)=0.0048

N_(p)=11

Total number of periods=3597 mixed periodsFor these grating parameters, a mode spacing of Δλ₁=4.7 nm was achieved.

As another example of a SSG, the reflectance spectrum illustrated inFIG. 4B was obtained using the following 3-step modulated superstructuregrating parameters:

Duty cycles=[0.5 0.5 0.5]

Periods=[228.2 230 231.8] nm N_(sub)=[131 130 129]

Λs=(29.894+29.9+29.902)=89.696 μmn_(H)=3.3757; n_(L)=3.3709;Δn=n_(H)−n_(L)=0.0048

N_(p)=11

Total number of periods=4290 mixed periodsFor these grating parameters, a mode spacing of Δλ₂=4.0 nm was achieved.

FIG. 4C illustrates an overlay of the reflectance spectra shown in FIG.4A and FIG. 4B. FIG. 4D illustrates constructive interference betweenthe reflectance spectra shown in FIG. 4A and FIG. 4B. The first andsecond modulated grating reflectors are designed to provide differentpeak spacings such that only a single peak is aligned. Thus, only onecavity mode is selected for lasing. As described below, the single peakcan be widely tuned over wavelength space based on thermal effect, freecarrier injection, or the like. Although embodiments of the presentinvention are illustrated in relation to operation and tunability around1550 nm, other wavelengths are available using appropriate semiconductorlaser materials.

Thus, implementations of the silicon hybrid tunable laser of the presentinvention was capable of tuning over the substantially entire wavelengthrange of interest. Tuning can be achieved, as described more fully belowusing several techniques including thermal tuning Referring once againto FIGS. 4A and 4B, the illustrated embodiment is operable over a rangeof temperatures including 40° C. Tuning of the laser wavelength can beconsidered as follows: the comb of wavelengths illustrated in FIG. 4A iscreated by the first modulated grating reflector 12 illustrated in FIG.1A. The comb of wavelengths illustrated in FIG. 4B is created by thesecond modulated grating reflector 14 illustrated in FIG. 4B. Theoverlay of the first comb and the second comb is illustrated in FIG. 4Cand demonstrates the combination of the wavelengths obtained from thefirst modulated grating reflector 12 and the second modulated gratingreflector 14. The constructive interference between the two wavelengthcombs is illustrated in FIG. 4D, with substantially a single peak in thereflectance profile. The one strong reflection peak thus produces thesingle laser mode, which is the only mode supported by the combinedreflectances. In an embodiment, the spectrum illustrated in FIG. 4D willbe present as the output of the optical coupler 16 provided to the phaseadjustment section 18.

FIG. 5A is a plot illustrating operating wavelength as a function oftemperature change according to an embodiment of the present invention.As illustrated in FIG. 5A, the operating wavelength shifts as a functionof temperature in a substantially linear manner. As will be evident toone of skill in the art, the shift in wavelength of the reflection peakas a function of temperature (and index of refraction) results in theshift in operating wavelength.

FIG. 5B illustrates wavelength shifting of a reflectance spectrum as afunction of index of refraction according to an embodiment of thepresent invention. For a nominal index (Δn=0), the peaks of the comb arelocated at a first set of wavelengths. As the index of refraction isshifted, for example, by thermal tuning, the comb shifts to a new set ofwavelengths as illustrated by the combs associated with Δn=0.003 andΔn=0.006. Thus, embodiments of the present invention provide fortunability of silicon photonics in which tuning is accomplished usingthe thermo optic (TO) effect of silicon. The TO coefficient of siliconis approximately

C _(TO)=2.4×10⁴ K ⁻¹

over the temperature range up to 650° C. In the embodiments describedherein, a conventional silicon ridge waveguide was used for waveguidingso that the TO is considered to be in the same range as the value givenabove. The index of refraction due to the TO effect can be expressed as:

Δn=C _(TO) ΔT.

Thus, for a temperate change of about 40° C., a change in the index ofrefraction of about 0.0096 can be provided for silicon material. Asillustrated in FIG. 5B, this translates to a change of about 4 nm inlaser wavelength change. It should be noted that the dynamic tuningrange for each mode can be adjusted by increasing the number ofsuper-periods (N_(p)).

In addition to thermal tuning, embodiments of the present invention canutilize current tuning based on the Kramer-Kronig relation.

FIG. 6 is a simplified flowchart illustrating a method of operating ahybrid integrated laser according to an embodiment of the presentinvention. The method 600, which may be utilized in operating a tunablelaser, includes tuning a first wavelength selective device (e.g., afirst modulated grating reflector disposed in a silicon layer of an SOIwafer) (610) and tuning a second wavelength selective device (e.g., asecond modulated grating reflector disposed in the silicon layer of theSOI wafer) (612). The first wavelength selective device is characterizedby a first reflectance spectra including a first plurality ofreflectance peaks. The second wavelength selective device ischaracterized by a second reflectance spectra including a secondplurality of reflectance peaks. In a particular embodiment, a firstmodulated grating reflector includes a superstructure gratingcharacterized by a first wavelength spacing between modes and a secondmodulated grating reflector includes a superstructure gratingcharacterized by a second wavelength spacing between modes that isdifferent than the first wavelength spacing between modes. Thewavelength selective devices can include index of refraction adjustmentdevices such as thermal devices that enable the tuning functionalitythat is provided. In applications with thermal devices, temperaturesensors such as RTDs can be used to monitor and control thermal inputs.One of ordinary skill in the art would recognize many variations,modifications, and alternatives.

The method also includes generating optical emission from a gain mediumcomprising a compound semiconductor material (614) and waveguiding theoptical emission to pass through an optical coupler (616). The opticalemission may pass through a phase adjustment region. The method furtherincludes reflecting a portion of the optical emission having a spectralbandwidth defined by an overlap of one of the first plurality ofreflectance peaks and one of the second plurality of reflectance peaks(618), amplifying the portion of the optical emission in the gain medium(620), and transmitting a portion of the amplified optical emissionthrough an output mirror (622).

It should be appreciated that the specific steps illustrated in FIG. 6provide a particular method of operating a hybrid integrated laseraccording to an embodiment of the present invention. Other sequences ofsteps may also be performed according to alternative embodiments. Forexample, alternative embodiments of the present invention may performthe steps outlined above in a different order. Moreover, the individualsteps illustrated in FIG. 6 may include multiple sub-steps that may beperformed in various sequences as appropriate to the individual step.Furthermore, additional steps may be added or removed depending on theparticular applications. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives.

FIG. 7 is a simplified plan view illustrating a hybrid integratedtunable pulsed laser and a phase modulator according to an embodiment ofthe present invention. The combination of a tunable laser and amodulator provides a tunable pulsed laser system. It will be appreciatedthat the laser itself is typically operated CW and modulation of the CWlaser light by the optical modulator results in intensity variation inthe optical output. Thus, although the present specification discussespulsed laser operation, the overall device can be considered as a pulsedlaser, although possibly not in the same way that a laser engineer mightthink of a “classical” pulsed laser in which lasing action throughoutthe entire cavity is suppressed. As illustrated in FIG. 7, an opticalmodulator 710 and a waveguide section 730 for suppression of StimulatedBrillouin Scattering (SBS) are integrated with the hybrid-integratedtunable laser described herein. Modulation of the phase of the pulsedoptical signal prior to delivery to an optical fiber (not shown)optically coupled to the output mirror of the device illustrated in FIG.7 provides suppression of SBS in the optical fiber. In an embodiment,the refractive index of a waveguide section 730 is varied in apredetermined manner to suppress SBS in the optical fiber. Physicalmechanisms for creating time-dependent variation in the refractive indexusing the waveguide section may include, but are not limited to, thethermo-optic effect, electro-optic effect, and free carrier effects.

In some embodiments, the phase of the light is modulated by electricallycontrolling the refractive index of the optical waveguide section in theSBS suppression section. Through the application of a time-varyingsignal to the electrical elements controlling refractive index, atime-varying change in the effective optical length in wavelength unitsof the SBS suppression section is effected. This, as a consequence,results in a time varying phase at the output related to the appliedtime-varying electrical signal. The electrical signal is designed tobroaden the line-width of the optical source such that it is greaterthan the Brillouin linewidth, which might typically be about 20 MHz.Although a single optical modulator 710 and a single waveguide section730 are illustrated in FIG. 7, one or more optical modulators and/or oneor more waveguide sections useful as phase modulators can be utilizedaccording to embodiments of the present invention. Thus, embodiments ofthe present invention are suitable for use with advanced modulationformats.

After light passes through the gain chip 20, it is split into the twolegs 712 and 714 of the illustrated modulator, where the light in oneleg can be phase shifted with respect to the light in the other leg,enabling modulation of the light to be implemented. Elements to applythe phase shift to the light in one leg with respect to the other leg,such as electrodes, conductors, and the like, are not illustrated forthe purpose of clarity.

The optical modulator 710 illustrated in FIG. 7 is a Mach Zehndermodulator, but other optical modulators utilizing other modulationmethods, for example, amplitude modulation using absorption effects(e.g., the quantum-confined Stark effect) may be utilized according toalternative embodiments of the present invention. Therefore, although aMach Zehnder modulator is illustrated in FIG. 7, embodiments of thepresent invention are not limited to this particular implementation.

Utilizing the fabrication methods described herein, the opticalmodulator 710 and the waveguide section 730 may be directly integratedinto the silicon. In other embodiments, materials other than silicon areused in implementing the modulator and/or the waveguide section and canbe fabricated using composite bonding methods. Examples of othermaterials suitable for inclusion in the modulator include ternary orquaternary materials lattice-matched to InP or GaAs, non-linear opticalmaterials such as lithium niobate, or the like.

In the embodiment illustrated in FIG. 7, the optical modulator and thewaveguide section can modulate the output produced from output mirror 21formed on the gain chip (i.e., an external modulation technique) or canbe operated as intracavity modulators (i.e., the optical modulator andthe waveguide section are intracavity optical elements), with an outputmirror provided at surface 720. Thus, both external modulation andinternal modulation techniques are included within the scope of thepresent invention. The output produced by the tunable laser with SBSsuppression is characterized by a tunable wavelength and pulsecharacteristics associated with the optical modulator and the waveguidesection.

In one implementation, the output mirror 21 is formed by the facet ofthe gain chip, so that the output is upstream of the optical modulator710. In other embodiments, the output mirror is positioned between theoptical modulator 710 and the waveguide section 730.

Thus, in an embodiment of the present invention, an optical phasemodulator is included to provide suppression of SBS in the optical mediacoupled to the device. In another embodiment of the present invention,electronics are provided to drive and control all or a subset of theoptical devices with electrical input or output signals. In otherembodiments, further optical devices with or without their associatedelectronics, such as monitor photodiodes for various sections of theoptical path, are included on the silicon photonic chip.

It should be noted that embodiments of the present invention provide forcombinations of amplitude modulation, phase modulation, and polarizationmultiplexing techniques. As will be evident to one of skill in the art,advanced modulation techniques encode information in both amplitude andphase and RF signals can be sent directly on optical carriers withoutconversion to the digital domain. Thus, embodiments of the presentinvention provide methods and systems suitable for such advancedmodulation techniques.

FIG. 8 is a simplified flowchart illustrating a method of operating atunable pulsed laser according to an embodiment of the presentinvention. The method includes tuning a first modulated gratingreflector (810) and tuning a second modulated grating reflector (812).The first modulated grating reflector is characterized by a firstreflectance spectra including a first plurality of reflectance peaks andthe second modulated grating reflector is characterized by a secondreflectance spectra including a second plurality of reflectance peaks.The method also includes generating optical emission from a gain mediumcomprising a compound semiconductor material (814), waveguiding theoptical emission to pass through an optical coupler (816), andreflecting a portion of the optical emission having a spectral bandwidthdefined by an overlap of one of the first plurality of reflectance peaksand one of the second plurality of reflectance peaks (818). The methodfurther includes amplifying the portion of the optical emission in thegain medium (820), transmitting a portion of the amplified opticalemission through an output mirror (822), optically modulating thetransmitted optical emission to form a pulsed optical output (824), andphase modulating the pulsed optical output (826). Phase modulation canprovide a time varying phase profile for the output.

In an embodiment, the first modulated grating reflector and the secondmodulated grating reflector are disposed in a silicon on insulatorwafer. As an example, the silicon on insulator wafer can include asilicon substrate, an oxide layer disposed on the silicon substrate, anda silicon layer disposed on the oxide layer. The first modulated gratingreflector and the second modulated grating reflector can be disposed inthe silicon layer. In some embodiments, the phase adjustment isprovided.

It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this applicationand scope of the appended claims.

What is claimed is:
 1. A tunable pulsed laser comprising: a substratecomprising a silicon material; a gain medium coupled to the substrate,wherein the gain medium includes a compound semiconductor material; anoptical modulator optically coupled to the gain medium; a phasemodulator optically coupled to the optical modulator; a waveguidedisposed in the substrate and optically coupled to the gain medium; afirst wavelength selective element characterized by a first reflectancespectrum and disposed in the substrate; a second wavelength selectiveelement characterized by a second reflectance spectrum and disposed inthe substrate; an optical coupler disposed in the substrate and joiningthe first wavelength selective element, the second wavelength selectiveelement, and the waveguide; and an output mirror.
 2. The tunable pulsedlaser of claim 1 wherein: the first wavelength selective elementcomprises a first modulated grating reflector; and the second wavelengthselective element comprises a second modulated grating reflector.
 3. Thetunable pulsed laser of claim 2 wherein the first modulated gratingreflector comprises a superstructure grating characterized by a firstwavelength spacing between modes.
 4. The tunable pulsed laser of claim 3wherein the second modulated grating reflector comprises asuperstructure grating characterized by a second wavelength spacingbetween modes different than the first wavelength spacing between modes.5. The tunable pulsed laser of claim 1 wherein the silicon materialcomprises a silicon on insulator wafer.
 6. The tunable pulsed laser ofclaim 5 wherein the silicon on insulator wafer comprises a siliconsubstrate, an oxide layer disposed on the silicon substrate, and asilicon layer disposed on the oxide layer, wherein the first wavelengthselective element and the second wavelength selective element aredisposed in the silicon layer.
 7. The tunable pulsed laser of claim 1wherein the optical modulator and the phase modulator comprise externalcavity optical elements.
 8. The tunable pulsed laser of claim 1 wherein:the first wavelength selective element comprises a first index ofrefraction adjustment device; and the second wavelength selectiveelement comprises a second index of refraction adjustment device.
 9. Thetunable pulsed laser of claim 8 wherein: the first index of refractionadjustment device comprises a thermal device; and the second index ofrefraction adjustment device comprises a thermal device.
 10. The tunablepulsed laser of claim 8 wherein the first wavelength selective elementfurther comprises a first temperature sensor; and the second wavelengthselective element further comprises a second temperature sensor.
 11. Thetunable pulsed laser of claim 1 wherein the optical modulator comprisesa Mach-Zehnder modulator.
 12. The tunable pulsed laser of claim 1further comprising a phase adjustment section optically coupled betweenthe waveguide and the optical coupler.
 13. The tunable pulsed laser ofclaim 1 further comprising a second phase adjustment section operable tomodify an optical phase in at least one of the first wavelengthselective element or the second wavelength selective element.
 14. Amethod of operating a tunable pulsed laser, the method comprising:tuning a first modulated grating reflector, wherein the first modulatedgrating reflector is characterized by a first reflectance spectraincluding a first plurality of reflectance peaks; tuning a secondmodulated grating reflector, wherein the second modulated gratingreflector is characterized by a second reflectance spectra including asecond plurality of reflectance peaks; generating optical emission froma gain medium comprising a compound semiconductor material; waveguidingthe optical emission to pass through an optical coupler; reflecting aportion of the optical emission having a spectral bandwidth defined byan overlap of one of the first plurality of reflectance peaks and one ofthe second plurality of reflectance peaks; amplifying the portion of theoptical emission in the gain medium; transmitting a portion of theamplified optical emission through an output mirror; opticallymodulating the transmitted optical emission to form a pulsed opticaloutput; and phase modulating the pulsed optical output.
 15. The methodof claim 14 wherein the first modulated grating reflector and the secondmodulated grating reflector are disposed in a silicon on insulatorwafer.
 16. The method of claim 15 wherein the silicon on insulator wafercomprises a silicon substrate, an oxide layer disposed on the siliconsubstrate, and a silicon layer disposed on the oxide layer, wherein thefirst modulated grating reflector and the second modulated gratingreflector are disposed in the silicon layer.
 17. The method of claim 15wherein phase modulating the pulsed optical output comprises producing atime varying phase profile.
 18. The method of claim 14 wherein the firstmodulated grating reflector comprises a superstructure gratingcharacterized by a first wavelength spacing between modes.
 19. Themethod of claim 18 wherein the second modulated grating reflectorcomprises a superstructure grating characterized by a second wavelengthspacing between modes different than the first wavelength spacingbetween modes.
 20. The method of claim 14 further comprising phaseadjusting the optical emission.